Positively Charged Gold Quantum Dots: An Nanozymatic "Off-On" Sensor for Thiocyanate Detection.

The concentration of thiocyanate (SCN) in bodily fluids is a good indicator of potential and severe health issues such as nasal bleeding, goiters, vertigo, unconsciousness, several inflammatory diseases, and cystic fibrosis. Herein, a visual SCN sensing method has been developed using the enzyme-like nature of positively charged gold quantum dots (Au QDs) mixed with 3,3',5,5'- (TMB) and hydrogen peroxide (HO. This research also reports a new method of synthesizing positively charged Au QDs directly from gold nanoparticles through a hydrothermal process. Microscopic imaging has showed that the Au QDs were 3-5 nm in size, and the emission wavelength was at 438 nm. Au QDs did not display any enzyme-like nature while mixed up with TMB and HO. However, the nanozymatic activity of Au QDs appeared when SCN was included, leading to a very low detection limit (LOD) of 8 nM and 99-105% recovery in complex media. The steady-state kinetic reaction of Au QDs showed that Au QDs had a lower Michaelis-Menten constant () toward HO and TMB, which indicates that the Au QDs had a higher affinity for HO and TMB than horseradish peroxidase (HRP). A mechanism study has revealed that the scavenging ability of hydroxyl (•OH) radicals by the SCN group plays an important role in enhancing the sensitivity in this study. The proposed nanozymatic "Off-On" SCN sensor was also successfully validated in commercial milk samples.